Conventionally, radiographic images such as X-ray images have been commonly utilized for diagnoses of medical conditions of a patient. In particular, conventional radiographic images have been improved to achieve a high reliability, superior cost performance, a high sensitivity, and a high image quality. Thus, they are still utilized at medical facilities all over the world as an image pick-up system.
However, the image information in conventional radiographic images is so-called analogue image information, and it is impossible to perform free image processing and image transmission as compared to digital image information which has been ever developing in recent years.
Therefore, in recent years, a radiographic image detector system such as a computed radiography (CR) and a flat-panel type radiation detector has come into use. Since these new systems directly obtain a digital radiographic image and directly display the image on an image display device such as a cathode ray tube and a liquid crystal panel, it is not necessary to use photographic film. As a result, these digital X-ray image detector systems have decreased the need of image formation by silver salt photography techniques and significantly improved convenience of diagnostic works at hospitals and clinics.
CR has come into practical use in the medical field and is, at present, one of the digital technologies for X-ray images. However, the sharpness is not sufficient nor is the spatial resolution, and CR has not achieved an image quality of a screen-film system. In addition, a flat plate X-ray detector system (FPD) employing a thin film transistor (TFT), described in “Amorphous Semiconductor Usher in Digital X-ray Imaging” by Jhon Rawlands, Physics Today, 1997 Nov., p. 24, and “Development of a High Resolution, Active Matrix, Flat-Panel Imager with Enhanced Fill Factor” by L. I. Anthonuk, SPIE, 1997, vol. 32, p. 2, has been developed as a further new digital X-ray image technology. A FPD is characterized by being smaller in size than a CR and being superior in image quality of image pick-up at a high dose. However, a FPD had a problem that the SN ratio decreased at image pick-up at a low dose and prevented a sufficient image quality due to electric noise derived from a TFT and a circuit.
A FPD utilizes a scintillator plate which is prepared from an X-ray fluorescent material provided with a property to convert radiation into visible light, and it is necessary to utilize a scintillator plate having a high emission efficiency to improve the SN ratio in image pick-up at a low dose. Generally, an emission efficiency of a scintillator plate is determined by the thickness of the fluorescent layer and the X-ray absorption coefficient of a fluorescent material, however, the thicker the fluorescent layer, the lower the sharpness due to scattering of emission light in a fluorescent layer. Therefore, the layer thickness is determined after sharpness required for the image quality.
Particularly, since cesium iodide (CsI) has a relatively high conversion rate from X-ray to visible light and the fluorescent material can be easily formed into a columnar crystal structure by vacuum evaporation, scattering of emission light in a crystal is depressed by an optical guide effect, whereby the thickness of a fluorescent layer can be made thicker.
However, since CsI alone shows a low emission efficiency, patent literature 1, discloses a technology, in which a mixture of CsI and sodium fluoride (NaI) is evaporated at an arbitrary mole ratio to accumulate sodium activated cesium iodide (CsI:Na) on a substrate, followed by annealing to improve the visible conversion efficiency. Such material is utilized as an X-ray fluorescent material.
Further, recently, for example patent literature 2, discloses a technology, in which thin layers are formed comprising CsI by vacuum evaporation and an activator substance such as indium (In), thallium (Tl), lithium (Li), potassium (K), rubidium (Rb) and sodium (Na) by spattering to prepare an X-ray fluorescent material.
However, the X-ray fluorescent material prepared by a method described in patent literature 1 or a method described in patent literature 2, still have low emission efficiency by radioactive irradiation and require further improvement.
[Patent literature 1] Examined Japanese Patent Application Publication No. 54-35060
[Patent literature 2] Japanese Patent Publication Open to Public Inspection No. 2001-59899